Easier said than done. The code of Rails was also publicly available, but getting from a hello world app to diving into a hard core codebase such as that of Rails is a huge step. Understanding framework code is quite different from understanding app code as well.

I agree. I didn’t say it would be easy, though. ☺
In fact, understanding “magic” is bloody difficult. It it weren’t so, then it wouldn’t be “magic”.

It’s also worth observing that Meteor is not presented as a “framework”. The comparison with Rails was introduced here, on HN. They say it’s “a set of new technologies”. I guess this implies loose coupling, which is a healthy paradigm when applied to components/modules/objects of a development ecosystem.

If only Meteor were nothing more than a proof of concept, it would still be mind-blowing.